The present invention relates to a method for determining at least one operational transmit power over a physical channel for respective ones of at least one tone.
Such a method is already known in the art, e.g. from the article entitled “Distributed Multi-user Power Control for Digital Subscriber Lines”, from Wei YU, Georges GINIS and John M. CIOFFI, published in the IEEE Journal of Selected Areas in Communications (J-SAC) of June 2002.
Spectrum management and power control are central issues in the design of interference-limited multi-user digital communication systems, such as Digital Subscriber Line (DSL) systems.
As the demand for higher data rates increases, spectrum management and power control emerge as central issues for the following two reasons: first, high-speed DSL systems are evolving toward higher frequency bands, where the crosstalk problem is more pronounced. Second, remotely deployed DSL can potentially emit strong crosstalk into neighboring lines.
FIG. 1 illustrates the latter issue. 3 transceiver unit pairs RT1/CP1, CO1/CP2 and CO2/CP3 are connected via twisted pairs L1, L2 and L3 respectively. The twisted pairs L1, L2 and L3 are bundled together in the binder B on the way to the central office CO. Because of their close proximity, the lines create electromagnetic interference into each other. Near-end crosstalk (NEXT) refers to crosstalk created by transmitters located on the same side as the receiver. Far-end crosstalk (FEXT) refers to crosstalk created by transmitters located on the other side. NEXT is usually much stronger than FEXT. To avoid NEXT, DSL makes use of frequency division multiplexing, wherein upstream (from customer premises) and downstream (to customer premises) signals are assigned distinct frequency bands.
In order to shorten the loop length with the purpose of increasing the data rate, the transceiver unit RT1 is deployed closer to the customer premises CP1, e.g. by means of an optical fiber OF. This is referred to as remotely or RT deployed DSL, as opposed to centrally or CO deployed DSL.
The signal from the transceiver unit CO1 is attenuated to a certain extent when it starts coupling with the line L1 in the binder B, thereby creating a weak FEXT F12. On the contrary, the signal from the transceiver unit RT1 is much stronger when it starts coupling with the line L2, thereby creating a stronger FEXT F21.
Several power control methods have been proposed in the Literature.
The cited document discloses a power control method, wherein each transceiver unit allocates power by waterfilling against the background noise and interference from other transceiver units. The power allocation of one transceiver unit affects the interference seen by other transceiver units. This in turn affects their power allocation so there is a coupling between the power allocation of the different users. Iterative waterfilling employs an iterative procedure whereby each transceiver unit waterfills in turn until a convergence point is reached.
The disclosed power control method is a realization of dynamic spectrum management. It adapts to suit the direct and crosstalk channels seen by the transceiver units in each specific deployment. The result is a much less conservative design hence higher performance.
Yet, the disclosed power control method leads to transmit Power Spectral Density (PSD) which may exceed the spectral masks defined in DSL standards. Hence, it does not satisfy spectral compatibility rules under method A. Instead, it relies on method B tests to ensure compatibility. These tests are highly complex. Furthermore, it is unclear whether spectral compatibility of iterative waterfilling under method B can be guaranteed for all deployment scenarios.
An other deficiency of the disclosed power control method is that it essentially implements flat Power Back-Off (PBO) over short loops, such as those seen on RT deployed DSL. In this scenario, it inherits all of the problems associated with flat PBO.